1. Field of the Invention
The present invention relates to a toner to be used in a recording method such as an electrophotographic method, an electrostatic recording method, a magnetic recording method, or a toner jet method.
2. Description of the Related Art
An electrophotographic method involves: forming an electric latent image on a photosensitive member by any one of the various means; developing the latent image with toner to form a toner image; transferring the toner image onto a recording material (transfer material) such as paper; and fixing the toner image on the recording material with heat or pressure to provide a print or copied article.
With the advent of developed computers and developed multimedia, means for outputting a full-color image having additionally high definition has been recently demanded in a wide variety of fields ranging from offices to households. Heavy users require such high durability that image quality does not reduce even after copying or printing on a large number of sheets. In contrast, in a small office or household, from the viewpoints of space savings and energy savings, the following properties have been demanded while the acquisition of a high-quality image is attained: a reduction in size of an apparatus, the recycling of waste toner or the prevention of the production of waste toner (the removal of a cleaner), a reduction in fixation temperature, and image gloss for corresponding to photographic image quality.
The viscoelastic characteristic and melt viscosity of toner have been discussed from the viewpoint of compatibility between the durability and fixing performance of the toner. Since toner generally receives a mechanical frictional force in a developing assembly to deteriorate the toner, an improvement in viscoelastic characteristic or melt viscosity of the toner is advantageous for the suppression of the deterioration. However, the viscoelastic characteristic or melt viscosity of the toner must be lowered in order that low-temperature fixation or image gloss may be realized by curtailing an energy consumption in a fixing step. In addition, a reduction in viscoelastic characteristic or melt viscosity of the toner not only provides obstacles to developing property and transferring property but also reduces the storage stability of the toner in an environment having a temperature around 50° C. On the other hand, a wax component in each particle of the toner preferably bleeds as instantaneously as possible (bleeding performance is preferably as high as possible) in the fixing step because the releasing performance of the toner from a fixing roller becomes favorable. However, when the wax component bleeds in a developing step, developing performance may deteriorate owing to insufficient charging of the toner due to the wax component. Investigations have been conducted on an approach to achieving compatibility between durability and fixing performance, which are mutually contradictory as described above.
Some attempts to achieve compatibility between durability and fixing performance are each based on attention paid on the DSC curve of toner in a differential scanning calorimeter (DSC). A toner containing at least a binder resin and a colorant and having the following characteristic has been proposed (Patent Document 1): at least one exothermic peak is present near the glass transition point of the binder resin in a second temperature increase process of the DSC curve of the toner measured with a differential scanning calorimeter. Although the fixing performance of the toner can be improved by the approach, the approach generally requires a further improvement in consideration of durability related to developing property at temperatures around room temperature.
On the other hand, when one wishes to achieve compatibility between the durability and fixing performance of toner while taking the internal structure of each particle of the toner into consideration, the durability and fixing performance of any one particle of the toner must be discussed, and the hardness (microscopic compression hardness) of any one particle of the toner can be an effective indication: the hardness (microscopic compression hardness) of any one particle of the toner represents the extent to which the toner particle deforms (elastically or plastically). Therefore, the microscopic compression hardness of the toner can be an effective indication of transferring performance as well as the durability and the fixing performance in a transferring step where a toner particle may deform owing to a pressure applied to the particle like contact transfer.
For example, the following has been disclosed (Patent Documents 2 and 3): in a capsule toner (of a core-shell structure) constituted of a thermofusible core (core) formed of a thermoplastic resin having a low glass transition point and an outer shell (shell) mainly formed of amorphous polyester, a relationship between a displacement by which one particle of the toner is compressed upon application of a load to the particle and the load is specified in a specific range, whereby compatibility among low-temperature fixability, offset resistance, and durability can be achieved. The capsule toner is effective in a heat-pressure fixing step because the toner is of such a structure that the core having a low glass transition point is coated with a relatively thick shell layer. However, the capsule toner has difficulty in satisfying low-temperature fixability or high image gloss in a light-load fixing step.
In addition, the following has been disclosed (Patent Document 4): an association method toner in which a high-molecular weight body and a low-molecular weight body are caused to exist in a toner binder resin so that each toner particle is provided with a certain hardness is excellent in durability without involving any detrimental effect caused by a triboelectric charging action due to a toner carrying member and a toner layer control member in a non-magnetic, one-component developing system. The storage stability and hot offset resistance of the association method toner, which is a toner obtained through a step of subjecting a resin particle and a colorant particle to salting out and melt adhesion, may reduce because the structure of the resin particle is controlled so that the molecular weight of the resin of which each layer is constituted may reduce from the central portion of the particle to the surface layer of the particle.
Further, it has been disclosed that, when a toner having the following characteristics is used, the toner easily splits in a fixing step, but is excellent in durability in a developing device and provides stable charging property (Patent Document 5): a load-displacement curve obtained by subjecting the particles of the toner to a microscopic compression test has a point of inflection, and the load at the point of inflection is larger than a load which the toner receives in a developing assembly. Although the toner can satisfy fixing performance in the fixing step, the toner cannot satisfy low-temperature fixability in corresponding to the reduction of the load or an increase in speed in the fixing step, and, furthermore, the toner hardly provides high image gloss.
As described above, a large number of investigations on compatibility between durability and fixing performance taking the internal structure of a toner particle into consideration have been conducted. However, in today's circumstances where an additional increase in speed and a full-color image having additionally high definition are requested, such investigations are still insufficient, and a toner capable of sufficiently satisfying high durability, high transferring performance, and, furthermore, storage stability while maintaining good fixing performance and high image gloss has been demanded.
[Patent Document 1] JP 2004-184561 A
[Patent Document 2] JP 03003018 B
[Patent Document 3] JP 03391931 B
[Patent Document 4] JP 2004-109601 A
[Patent Document 5] JP 2005-300937 A